"I can't believe it's not better!": Dermatology look-alikes (Proceedings)


Here's what I like about skin: it's on the surface of the body. This minimizes the risk of me causing a hemorrhagic event when I do a biopsy of my organ of specialty. What I don't like is the skin has limited ways in which it manifests disease, so the symptoms of many diseases overlap.

Here's what I like about skin: it's on the surface of the body. This minimizes the risk of me causing a hemorrhagic event when I do a biopsy of my organ of specialty. What I don't like is the skin has limited ways in which it manifests disease, so the symptoms of many diseases overlap. This is a discussion of syndromes with similar clinical appearances, yet different therapies, and how to differentiate between them.

Example 1: When a bump is more than a bump: Folliculitis vs. Hives in the Short-Coated Dog

      Clinical Appearances

Superficial bacterial infection of the skin usually often originates in hair follicles. The first clinical change associated with this is change in orientation of the hair. In short-coated breeds (e.g., dachshunds, boxers) this looks like as a small group (<4 mm diam) group of raised hairs. Many owners and some veterinarians interpret this change as "hives". However, this "hive" lasts for 24-48 hours or more, after which the hair falls out leaving moth-eaten alopecia. Pyoderma lesions are most common on the trunk and are a common symptom of allergic dermatitis.

A hive or wheal (urticaria) is the manifestation of a type 1 hypersensitivity reaction and is comprised of edema in the dermis and epidermis. It has a sudden onset and frequently self-resolves in 24 hours if not treated. The lesions vary in size from a few millimeters to 1.5 to 2 cm. Lesions can occur on any part of the body, with the head and face more frequently affected. Some urticarial reactions are marked enough to cause subsequent alopecia, but this is not the rule. Self-trauma can cause secondary infection and alopecia.


If the clinical differences do not make diagnosis clear, these lesions can be differentiated either by biopsy or response to an appropriate course of antibiotics.

      Bottom Line: Don't always believe your clients when they say "my dog has hives."

Example 2: The Devious Epidermal Collarette: Canine Pyoderma vs. Dermatophytosis

      Clinical Appearances

Pyoderma: The primary lesion of canine superficial pyoderma / bacterial folliculitis is a papule or pustule which quickly ruptures, leaves scale, causes hair loss and can form an epidermal collarette. This is a peripherally expanding lesion with a leading edge of peeling skin and erythema. The center becomes hyperpigmented during the healing phase. To many pet owners, this looks like "ringworm". Most pyoderma lesions occur on the trunk, fewer on the extremities. Lesions are mild to moderately pruritic.

      Dermatophytosis: Depending upon the offending fungal species and the host's immune response, the primary lesion of a dermatophyte infection is alopecia or alopecia with scaling and papules. It can occur anywhere on the body and may or may not be pruritic.


Cytology is helpful if it reveals intracellular bacteria. However, they are easily missed. Better to do a fungal culture, start treatment with antibiotic appropriate for staphylococcus (cephalexin, amoxi + clavulonic acid, cefpodoxime, cefovicin), and monitor response.

      Bottom line: "If it looks like ringworm, it probably isn't." It's probably pyoderma.

Example 3: Fungal-induced acantholysis; Canine Trichophyton mentagrophytes infection vs. Pemphigus foliaceus / erythematosus

      Clinical appearances

Trichophyton: This superficial fungal infection causes alopecia, papules / pustules / furunculosis, scaling, alopecia, and hyperpigmentation starting on the face and extremities, then progressing to the body. Infectious causes of facial dermatitis usually leave the nasal planum alone. Lesions are often asymmetrical and may be mildly to markedly pruritic. In my experience, Jack Russell Terriers seem predisposed to this condition.

      Pemphigus: Primary lesion is a pustule that quickly ruptures, leaving layers of scale, crusts, erythema and alopecia. The face and feet are the most commonly affected areas, but it can affect the whole body. Signs may wax and wane. May see depigmentation, ulceration, and loss of architecture of the nasal planum. Lesions are often symmetrical and can be very pruritic.


It is best to do both biopsy and fungal culture to differentiate between these two conditions.

Fungal culture of hairs from affected areas is needed to identify the fungus involved. I have missed dermatophyte on culture that was seen on biopsy and vice verse.

The lesion of pemphigus foliaceus is a pustule with liberated nucleated keratinocytes (acanthocytes). Histopathology stains for infectious agents and a carefully performed culture of a PF pustule are negative.

Dermatophytosis creates inflammation directed at infected hair follicles. Additionally, T. mentag can produce keratolytic enzymes that cause acantholysis, creating lesions similar to pemphigus. If special stains for infectious agents are not done on a biopsy specimen, the fungal elements can be overlooked.

      Bottom line: In canine patient with alopecia and inflammation especially focused around the face, do a biopsy and fungal culture and make sure the pathologist does special stains for fungi.

Example 4: Antibiotic responsive lupus? Canine Mucocutaneous Pyoderma vs. Discoid lupus erythematosus (DLE)

      Clinical appearance:

Mucocutaneous Pyoderma (MCP): This condition presents as erythema, ulceration, crusting and sometimes fissuring on the lip margins and commissures. It can also affect the margins (dorsal and ventral) of the nasal planum. German shepherd dogs seem predisposed. This problem, to me, seems more common in dogs with underlying hypersensitivities and flares concurrent with their allergies.

DLE: In its most common form, DLE causes depigmentation, "smoothing", and ulceration / crusting of the nasal planum. It can less often affect eyelid margins. Again, this is common in German shepherd dogs but can be seen in any breed dog (rare in brachycephaleic dogs).

There has been discussion amongst dermatologists regarding nasal dermatoses that resemble DLE clinically and histopathologically, yet respond to antibiotic therapy only. I have also seen the opposite.


DLE is diagnosed by biopsy and lack of response to antibiotics; MCP is diagnosed based on response to therapy ± biopsy.

      Bottom line: Erosions of the perioral area / nasal planum can have an infectious or immune-based origin. Histopathology may not clearly illuminate the cause. A 3-4 week long course an appropriate antibiotic (such as cephalexin) is justified prior to doing a biopsy. If response is poor, then biopsy to rule out DLE v. other nasal dermatoses (e.g., actinic changes, pemphigus, neoplasia) is warranted.

Example 5: Scrape every dermatology case Demodex vs. The Dermatology Text Book

      Clinical Appearance

My experience (sometimes painful) is that demodicosis can look like anything! Classic clinical appearance of demodicosis: asymmetrical alopecia most common on the face and feet. Comedones and secondary pyoderma (erythema, papules, crusting) are very common.

Other presentations: Demodex can cause marked scaling resembling an exfoliative dermatitis; just pustules on the ventrum; comedones and alopecia similar to endocrine dermatosis. It can cause follicular casts that we see in sebaceous adenitis (discussed next); it can "hide" behind pyoderma; it can cause furunculosis and draining tracts before you see significant hair loss; and it can cause ceruminous otitis with no other signs.


Skin scrapings; hair plucks from areas of very thickened skin; biopsy.

      Bottom line: most, if not all, dermatology cases deserve skin scrapes or hair plucks.

Example 6: Other things that cause follicular casts

Granulomatous Sebaceous Adenitis (GSA) vs. Pyoderma and hypothyroidism

GSA is the loss of the sebaceous glands within the skin. The condition has a genetic basis. We do not know the exact pathogenesis of the glandular destruction; it does not appear to be a straight-forward immune-mediated issue and may be in part a disorder of keratinization.

      Clinical appearance

The signs of GSA vary between breeds (hair coats?). In short-coated breeds, the main lesion is circular alopecia with scaling (much like pyoderma). In Standard Poodles, the breed that defined this problem, GSA causes marked scaling and follicular casts on the head and body. For the Akita, GSA causes erythemic, oily, alopecic areas with papules, pustules, scaling, follicular casts. They may also get a "rat tail". In Samoyeds and cross-breeds, it may cause diffuse truncal alopecia with scaling and follicular casts.

Hypothyroidism: Lack of thyroid hormone can cause a thin hair coat, seborrhea, "rat tail", and recurrent pyoderma.


Many of the scenarios described above sound like pyoderma, and GSA can be complicated by pyoderma. A course of antibiotics and blood work are the first steps. Equally important is a recheck 3 weeks later. If lesions are not resolved, if follicular casts remain, a skin biopsy is indicated. Demodex will also cause follicular casts, so don't forget the skin scrapes.

Example 7: Copy Cats: Pemphigus Foliaceus vs. Severe Allergic Dermatitis in the Cat

      Clinical Appearances

      Allergic Dermatitis: A common expression of hypersensitivity reaction in the cat is a military dermatitis. When secondary infection is present, pustules, more crusting, and severe pruritus can ensue. When the allergic reaction centers on the head and neck, this can mimic pemphigus foliaceus.

      Pemphigus: The primary lesion of PF is a very short-lived pustule, which quickly crusts after rupture. Subjectively, the crust from a PF lesion is thicker than a nasty case of military dermatitis. The lesions of PF are often worst on the head, but also affect nail beds and skin around the nipples. PF can be very pruritic and eosinophilic, confusing it with allergic dermatitis.


Plan A is biopsy and fungal culture. Make sure to include even dry crusts that fall off the skin in the formalin jar. Start antibiotics and at least an anti-inflammatory dose of corticosteroids pending the results.

Another approach is to do cytology of a fresh papule / pustule. If acanthocytes, non-degenerate neutrophils and eosinophils are seen, yet no bacteria, I am more suspicious of PF and recommend a biopsy to confirm the diagnosis. If I see even a few bacteria and some degenerate white blood cells, I treat this as a hypersensitivity case unless it progresses despite appropriate therapy.

      The Very Bottom Line: the skin has a limited ways of expressing itself and appearances of diverging clinical syndromes can overlap greatly. Most problems warrant skin scrapings and a course of an appropriate antibiotic prior to or while awaiting results of a biopsy.

Selected References

Cannon, A. Food allergy and nutritionally related skin disease. In: Thoday KL, Foil CS, Bond R eds/ Advances in Veterinary Dermatology Vol 4 Oxford: Blackwell Science Ltd, 2002; 228.

Parker WM, Yager JA. Trichophyton dermatophytosis--a disease easily confused with pemphigus erythematosus. Can Vet J. 1997 Aug;38(8):502-5.

Scott, Miller and Griffin. Muller and Kirk's Small Animal Dermatology, 6th ed.

Wiemelt SP, Goldschmidt MH, Greek JS, et al. A retrospective study comparing the histopathological features and response to treatment in two canine nasal dermatoses, discoid lupus erythematosus and Mucocutaneous pyoderma. Vet Dermatol 2004 Dec;15 (6):341-8

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